A grain tank of a combine harvester is frequently provided with an extension attachment that comprises several flaps pivotally linked to a frame to allow swiveling. Such a grain tank attachment is, for example, known from DE 10 2004 007 673 A1. When a combine harvester is travelling on the road, the flaps are swivelled inwards and lie flat on the opening of the grain tank so that the overall height of the combine harvester that is permissible on road traffic is not exceeded. During operation in the field, the flaps are in the swivelled-out position, in which they extend obliquely outwards and upwards from the frame. In this manner, they increase the volume of the grain tank, so that said tank can be emptied at longer intervals.
In order to make the intervals between two emptying sequences as large as possible, it seems desirable at first sight to provide an attachment with flaps which are as large and as widely spread as possible. By doing so, however, the problem arises that the width of the flaps, i.e., their dimension transversely to the swivel axis, is limited by the width of the grain-tank opening, above which they must be laid one on top of the other in the closed position. Moreover, the centre of gravity of the combine harvester is increasingly shifted upwards when the extension attachment is filled with grain in the course of a harvesting process, so that the combine harvester's stability on sloped ground decreases. An extension attachment that extends too far up beyond the grain tank therefore impairs the operational safety of the combine harvester.